1. Field of the Invention
The present invention relates to a method utilized in a wireless communication and communication device thereof, and more particularly, to a method for improving a cell change mechanism in a wireless communication system and communication device thereof.
2. Description of the Prior Art
Inter-radio-access-technology (Inter-RAT) mobility involves transfer of a connection between a mobile device and a network from one RAT to another RAT. Inter-RAT transfer may occur due to handover or cell reselection. In the case of the cell reselection, the UE transfer a connection between cells using different RATs, such as GSM (Global System for Mobile communications)/EDEG (Enhanced Data Rates for GSM Evolution), UMTS (Universal Mobile Telecommunications System), and LTE (Long-term evolution).
A cell change order procedure from a GERAN cell, as network controlled cell reselection, is initiated via a PACKET CELL CHANGE ORDER message including information of the target cell. A cell change order procedure from a GERAN cell, as network controlled cell reselection, is initiated via a PACKET CELL CHANGE ORDER message including information of the target cell. Upon reception of the PACKET CELL CHANGE ORDER message, the UE starts a timer T3174 and selects the target cell to establish a connection. If the timer T3174 expires prior to successful RRC connection establishment, the cell change order procedure is failed and the MS returns to the GERAN cell. In addition, the cell change order procedure is also failed when access to the target cell is denied (i.e., the MS receives an IMMEDIATE ASSIGNMENT REJECT, a PACKET ASSIGNMENT REJECT or, in a UTRAN cell, an RRC CONNECTION REJECT message).
A cell change order procedure from a UMTS (UTRAN) cell is initiated via a CELL CHANGE ORDER FROM UTRAN message including information of the target cell. Upon reception of the CELL CHANGE ORDER FROM UTRAN message, the MS starts a timer T309 and selects the target cell to establish a RRC connection. If the timer T309 expires prior to successful RRC connection establishment, the cell change order procedure is failed and the MS returns to the UMTS cell. In addition, the cell change order procedure is also failed when access to the target cell is denied. As can be seen from the above, the timers T3174 and T309 provide a period of validity for corresponding cell change order procedure.
In a cell change order procedure from other RAT (e.g. GSM/UMTS) to a LTE cell, or an E-UTRA (Evolved UMTS Terrestrial Radio Access) cell, the UE enters the LTE cell with an RRC_IDlE state and then performs a random access procedure for uplink synchronization with an eNB (evolved Node-B) of the LTE cell and for the RRC connection establishment procedure. The random access procedure shall be well known and corresponding details are omitted. If the cell change order procedure is failed, the UE returns to the source cell. The LTE system employs several timers for access barring. A timer T302 is used for barring RRC connections related to mobile originating/terminating calls or mobile originating signaling; a timer T303 is used for barring the RRC connections related to the mobile originating calls; a timer T305 is used for barring the RRC connections related to the mobile originating signaling. In the RRC connection establishment procedure, a LTE cell in a barred state rejects a RRC connection request by sending a RRC CONNECTION REJECT message. In this situation, the UE needs to set the timer T302 to a wait time included in the RRC CONNECTION REJECT message. In other words, the access barring can be reflected by the timer T302.
Due to unclear specification for the cell change order procedure to the LTE cell, the UE encounters four scenarios described as follows.
In the first scenario, the UE receives a cell change order when performing data transmission (e.g. when staying in a packet transfer mode of GERAN system), and thereby tries to establish a RRC connection with the target LTE cell as soon as possible. However, the target LTE cell is barred temporally due to access barring for mobile originating calls or mobile originating signaling. The access barring time is so long that the UE encounters a huge delay of data transmission after the cell change. Continuity/Quality of the in-use service is affected.
In the second scenario, a UE in a GERAN (GSM/EDEG Radio Access Network) cell in a packet transfer mode receives the PACKET CELL CHANGE ORDER message for cell change to an E-UTRA cell. The UE starts timer T3174 that is set to 15 seconds. The UE selects the E-UTRA cell and performs a random access procedure for sending a RRC CONNECTION REQUEST message. The UE successfully completes the contention resolution, i.e. the UE detects a Temporary C-RNTI (assigned in a Random Access Response) and finds corresponding UE Contention Resolution Identity in a received packet. The UE regards the cell change order procedure as successful and stops the timer T3174. However, the E-UTRA cell responds a RRC CONNECTION REJECT message including a wait time whose range is from 1 to 16 seconds. The UE starts the timer T302 set to the wait time value that is set to 8 seconds. The UE is not allowed to access the E-UTRA cell before the timer T302 expires. When the timer T302 expires, the UE perform the RRC connection establishment procedure again. The UE does not get the access permission from the E-UTRA cell due to another receipt of the RRC CONNECTION REJECT message including another wait time that is set to 10 seconds. Due to a barred state of the E-UTRAN, the wait times can be set to a so larger number that the total wait time (18 s) exceeds a time limit (15 s) of the timer T3174. This delays the UE access for data transmission.
In the third scenario, a UE in a GERAN cell in the packet transfer mode receives a PACKET CELL CHANGE ORDER message for cell change to an E-UTRA cell. The UE starts a timer T3174 that is set to 15 seconds. The UE selects the E-UTRA cell and performs a random access procedure for sending a RRC CONNECTION REQUEST message. The E-UTRA cell responds a RRC CONNECTION REJECT message including a wait time that is set to 2 seconds, indicating failure of the network controlled cell reselection. However, the wait time herein is so short that the UE can re-send the RRC CONNECTION REQUEST message soon. If the UE returns to the old cell according to the abovementioned description, the UE loses the chance to get a better data rate from the E-UTRA.
In the fourth scenario, a UE in the GERAN cell in the packet transfer mode receives a PACKET CELL CHANGE ORDER message for cell change to an E-UTRA cell. The UE starts timer T3174 that is set to 15 seconds and selects the E-UTRA cell to send a RRC CONNECTION REQUEST message. If the E-UTRA cell is in the barred state, the UE starts the timers T303 or T305 when accessing to the E-UTRA cell. In this situation, the UE cannot perform a random access procedure due to access barring of mobile originating calls (T303) or mobile originating signaling (T305). If T303 or T305 is larger than 15 seconds, the UE cannot access the E-UTRA cell before the timer T3174 expires. This means that the cell change procedure is inevitably failed if the T303 or T305 value is larger than the T3174 value. After T3174 expires, the UE returns to the GERAN cell and sends a PACKET CELL CHANGE ORDER FAILURE message. In the abovementioned situation, it is not necessary for the UE to wait for the T3174 expiry because access to E-UTRA keeps being prohibited during the running of the T3174.
In the prior art, the cell change order procedure is only considered failed when an access denying message (e.g. an IMMEDIATE ASSIGNMENT REJECT message) is received without consideration of the access barring time of the target cell, resulting in the long delay of UE data transmission.